1. Field of Invention
This invention relates to an absolute position optical displacement-sensing device.
2. Description of Related Art
Absolute position encoders use a scale structure-that allows the position of a readhead relative to a scale to be determined without requiring any reference to a given starting point along the,scale. A variety of absolute position encoders are known which use multiples information tracks along the scale.
In many conventional capacitive and inductive absolute position encoders, the scale structures include two or more tracks that have different periodic spatial wavelengths that, in combination, uniquely identify the position of the readhead relative to the scale within a limited distance. U.S. Pat. No. 5,886,519 discloses one embodiment of such an inductive absolute position encoder.
The 519 patent also discloses an embodiment that includes a track having a periodic spatial layout of the sensible elements and a separate, code, track that includes a series of sensible elements that are arranged in a pseudo-random sequence. In particular, for any subset of the sensible elements of this code track that are located within the bounds of the readhead sensor elements, the sensible elements form a unique binary, trinary or higher-modulus code word that uniquely identifies the position of the readhead relative to the scale to a first resolution, while the track having a periodic spatial layout identifies the position of the readhead relative to the scale to a finer resolution.
Similarly, many conventional optical position encoders use multiple information tracks along the scale, where, for each xe2x80x9cpositionxe2x80x9d along the scale, a code track, or a combination of code tracks, defines a unique value that uniquely identifies that position along the scale. Often, methods are also used which determine the position of a periodic track relative to the readhead to a resolution that is smaller than the resolution provided by the one or more code tracks alone. U.S. Pat. Nos. 3,882,482, 5,965,879 and 5,279,044, incorporated herein by reference, disclose such multi-code-track optical encoders.
U.S. Pat. No. 4,631,519, as well as the 044 patent, disclose embodiments of optical absolute position encoders that integrate the previously mentioned code track functions and periodic track functions into a single combined track. In such embodiments, the code elements are arranged non-periodically along the track, but each individual code element is nevertheless positioned to align with an underlying continuous periodic grid.
The inventors of this invention have determined that the optical encoder configurations discussed above have a number of disadvantages. Multiple track disadvantages include requiring overly-large-sized readheads, overly-wide scales and poor xe2x80x9cinformation densityxe2x80x9d in at least some of the tracks. Conversely, if multiple track configurations are made narrow, sensitivity to static and/or dynamic mounting misalignments and/or the need for more complicated readhead sensing or signal processing arises. Previous combined track optical encoder configurations also have a number of disadvantages. These disadvantages include limited scale lengths if high resolution is desired, relatively low resolution if a long scale length is desired, and/or the need for relatively more complicated readhead sensing or signal processing to achieve speeds, accuracies and resolution comparable to that achievable using a track of elements arranged periodically. Furthermore, it is relatively expensive to fabricate scales having code element sequences that are everywhere unique along the scale but which must nevertheless be precisely aligned with an underlying continuous periodic grid in order to provide accurate and high-resolution absolute measurements. An optical absolute position encoder that could avoid any one or more of these disadvantages would be useful.
This invention provides optical absolute position encoders that have relatively small readhead sizes and/or relatively narrow scale widths.
This invention further provides optical absolute position encoders that reduce the size of the readhead and/or the width of the scale by using an integrated scale track that integrates both periodic and code structures.
This invention separately provides optical absolute position encoders that are relatively insensitive to static and dynamic misalignments of the scale in a direction transverse to the measuring axis.
This invention separately provides optical absolute position encoders that have both long scale lengths and high resolution.
This invention further provides optical absolute position encoders that obtain long scale lengths and high resolution by using an integrated scale track that includes a code portion that allows for long scale lengths and a periodic portion that allows for high-resolution determinations of the relative position between the readhead and the scale.
This invention separately provides a scale for optical absolute position encoders that has an integrated scale track that includes both a code portion and a periodic portion.
This invention further provides a scale that has an integrated scale track where the code portion and the periodic portion occur alternately along the length of the scale.
This invention additionally provides a scale track where the code portions and the periodic portions that occur alternately along the scale are adjacent to one another.
This invention separately provides methods for determining the absolute position of a readhead of an optical absolute position encoder relative to a scale of the optical absolute position encoder from an image of an integrated scale track that includes both non-periodic code portions and periodic portions.
This invention further provides methods for determining the absolute position of the readhead relative to the scale by locating a code portion appearing within an image of the scale and determining a first-resolution relative position based on the code appearing in the located code portion.
This invention additionally provides methods for determining the absolute position of the readhead relative to the scale by locating a predetermined portion appearing within the image of the scale and determining a second-resolution relative position based on the location of the predetermined portion within the image of the scale.
This invention also provides methods for determining the absolute position of the readhead relative to the scale by determining an offset distance between a periodic portion appearing within the image and a periodic reference structure and determining a third-resolution relative position based on the offset distance.
In various exemplary embodiments of the scale according to this invention, the scale includes an integrated scale track extending along a measuring axis. The integrated scale track includes one or more periodic portions placed along the measuring axis, where each periodic portion has a plurality of periodic elements. The integrated scale track also includes non-periodic code portions, which are placed between the periodic portions. Each non-periodic code portion is a unique set or group of code elements and thus identifies a specific location along the scale. In various exemplary embodiments, each non-periodic code portion includes a predetermined portion that extends along the measuring axis. The predetermined portion may be a single element or space having predetermined characteristics, or a predetermined pattern of elements. In various other exemplary embodiments, a predetermined portion that extends along the measuring axis is provided on the scale adjacent to one end, or even both ends, of the non-periodic code portion. In any case, this predetermined portion allows the readhead signals arising from the code portions of the scale to be rapidly distinguished from the readhead signals arising from other portions of the scale.
In various other exemplary embodiments according to this invention, the periodic scale elements are placed along the scale to coincide with an underlying incremental pitch that is continuous and spatially synchronized between the periodic portions, even though the non-periodic code portions interrupt the continuity of the periodic elements along the measuring axis. In various other exemplary embodiments according to this invention, at least some of the individual code elements of the non-periodic code portions are narrower in a direction along the measuring axis than the periodic scale elements. In various other exemplary embodiments according to this invention, one set of the code elements may be spaced apart from an adjacent set of code elements up to a determined limit based on the dimensions of a detector array of the readhead and a magnification applied to the scale image by the readhead.
In various exemplary embodiments, each set of code elements in effect indicates the position or measurement value of a local datum feature to provide a coarse resolution absolute position value. The local datum feature is associated with a plurality of periodic scale elements. In various other exemplary embodiments according to this invention, a detector array of the readhead further determines the position of the local datum feature relative to the detector array to a higher resolution that is finer than the period of the periodic scale elements. In various other exemplary embodiments according to this invention, the detector array of the readhead further determines the position of at least some of the periodic scale elements relative to the detector array and/or the readhead to provide the highest resolution absolute position measurement. In various other exemplary embodiments according to this invention, the detector array pitch is chosen such that there are a plurality of detector elements for each periodic scale element imaged by the array. In various exemplary embodiments, the detector array pitch is chosen such that there are a plurality of detector elements for each code element imaged by the array.
In various exemplary embodiments of the absolute scale according to this invention, the sets of code elements form sequential code words along the scale. In various other exemplary embodiments according to this invention, the sequential code words directly indicate the corresponding position along the scale. In various other exemplary embodiments according to this invention, the sequential code words are converted to absolute position measurement values using a decoder look-up table.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.